Featured Image in Custom Post Type

I´m using the following code in functions.php:

add_action('init', 'create_post_type');
function create_post_type() {
    register_post_type('portfolio-item',
        array(
            'labels' => array(
                'name' => __('Portfolio'),
                'singular_name' => __('Portfolio-Item')
            ),
            'public' => true,
            'has_archive' => true,
            'rewrite' => array('slug' => 'portfolio-item')
        )
    );
}
add_theme_support('post-thumbnails');

Where may I put your stuff in?